1. Field of the Invention
The invention relates to a semiconductor wafer composed of monocrystalline silicon having a front side and having a rear side and having a denuded zone, which extends from the front side in the direction of the rear side, and having a region which adjoins the denuded zone and has a specific density of BMDs. BMDs (bulk micro defects) are precipitates of oxygen in the monocrystalline environment, so-called oxygen precipitates. The invention also relates to a method for producing the semiconductor wafer.
2. Description of the Related Art
A denuded zone is an area of the semiconductor wafer which encompasses the front side and, owing to its lack of defects, is well suited as an environment for the construction of electronic structures. A region having BMDs which adjoins a denuded zone is held in high regard because oxygen precipitates act as getter centers that keep impurities away from the denuded zone.
A denuded zone is usually produced by thermal treatment of a substrate wafer at high temperatures. An RTA treatment is particularly suitable, that is to say a thermal treatment referred to as rapid thermal anneal, since in the course thereof the substrate wafer is heated to a target temperature at a high rate of temperature rise and is cooled after a comparatively short time at a high rate of temperature reduction. The time expenditure for producing the denuded zone by means of an RTA treatment is comparatively short.
If the RTA treatment is carried out in a nitriding atmosphere, this fosters the formation of oxygen precipitates in proximity to the denuded zone, since vacancies are injected in the course of the nitriding and incite the nucleation of oxygen precipitates. A mixture of argon and ammonia is particularly suitable as an RTA atmosphere, since the target temperature of the RTA treatment can thus also be reduced in a range in which the frequency of occurrence of slip is significantly reduced. A corresponding method is described in US 2004/0053516 A1, for example.
An appropriate source of the substrate wafers required are single crystals composed of silicon, in particular, which were pulled in accordance with the CZ method. In this method, silicon is melted in a crucible composed of quartz and the single crystal grows at the end of a seed crystal which is dipped into the resultant melt and is raised. The crucible material is partly dissolved by the melt and in this way provides oxygen that is required later for forming oxygen precipitates in the substrate wafer.
Specific defects, the formation of which depend in particular on the ratio V/G during the production of the single crystal, make it more difficult to form a denuded zone or prevent a denuded zone from being formed. If the ratio V/G of pulling rate V and axial temperature gradient at the interface between melt and growing single crystal during the production of the single crystal is between a lower threshold and an upper threshold, the formation of such defects does not happen. The defect formation is caused by a specific supersaturation of point defects which is not achieved under the conditions mentioned. If the ratio V/G is somewhat greater than the lower threshold, silicon interstitials are dominant, and if the ratio is somewhat less than the upper threshold, vacancies are dominant as point defect type. Overall, three zones can be differentiated in which no harmful supersaturation of point defects prevails. The [Pi] zone, which is dominated by silicon interstitials, the [Pv] zone, which is dominated by vacancies, and the OSF zone, which is dominated by vacancies and in which stacking faults can form after oxidation in oxygen.
It is complex and not very economic to produce single crystals from which substrate wafers are obtained which consist completely of only one of the zones mentioned. In order to achieve that, the ratio V/G is permitted to fluctuate only within very narrow limits. These limits are easily exceeded, since the ratio V/G is usually not constant along the phase boundary. Therefore, it is more economic to utilize the corridor between the lower and upper thresholds to the furthest possible extent. A single crystal that is pulled under such conditions generally yields substrate wafers comprising a [Pi] zone and a [Pv] zone.
The inventors subjected substrate wafers having these properties to an RTA treatment under a mixture of argon and ammonia and established that there are considerable differences regarding the depth of the denuded zone between the center and the edge of the substrate wafer. Such an inhomogeneous radial course is unfavorable, particularly for applications in which the semiconductor wafer is ground back in the course of further processing to form electronic components. If the denuded zone extends too deeply into the interior of the semiconductor wafer, it can happen that the region in which the oxygen precipitates are formed is completely removed during back grinding. The semiconductor wafer then lacks the necessary getter centers. The inventors furthermore established that the density of the oxygen precipitates increases only moderately in the direction toward the center plane of the semiconductor wafer. The center plane of the semiconductor wafer is the virtual plane between the front side and the rear side of the semiconductor wafer. During back grinding, therefore, it can happen that although a region with oxygen precipitates is maintained, the density of the oxygen precipitates in the region is too low to manifest a sufficient getter effect.
US 2005/0054124 A1 describes a method which comprises a two-stage RTA treatment and makes accessible a semiconductor wafer having a denuded zone, the depth of which from the center as far as the edge is relatively constant. Furthermore, the semiconductor wafer has a region which adjoins the denuded zone and has a density of oxygen precipitates that is virtually constant in the direction toward the center plane of the semiconductor wafer.